if you add a manifest to your project - you will see correct bitmaps in xp and vista. I suppose you have this problem because images have low alpha values in invisible part, but I have not really checked.

I've suspended this issue, since this is how windows works.
Without manifest, windows doesn't support alpha images in an imagelist. Alphavalues < 50% will be masked in that case.
I keep this issue open to rethink if there is a solution for this in the future